I bought a 2GB Clip+ last week, and added a 16 GB card. I loaded about 225 albums onto it, which added up to about 3200 files. It appeared to work flawlessly with the original Sansa firmware.

Impressed as I was with the Sansa quality, yesterday I went out and bought a couple more 4GB Clip+ along with 32GB cards for each of them. My plan is to put specific 500 album collections on each of them.

I transferred files onto the card using the same card reader I used last week. Everything seemed to work fine. (Since the 32 GB card is nowhere near 32GB, I only managed to get 485 albums onto the Clip+ and card combined.)

But after the Sansa firmware did it's database refresh, about 185 of the albums are missing from the album list! The missing ones are randomly gone from the middle and end of the list. My total number of files is ~ 6700 with about 500 folders.

Searching this forum, I thought the Clip+ file limit was 8000. But mine fails with less than 7000.

Finally, is there a fix to the original firmware, or is Rockbox the only option?

The 8000 file limit in the sansa is approximate. It can be affected by folder structure, length of file names and tags AFAIK. There's been a number of reports of people getting substantially less than 8000 file to show. There was one person that reported they could only see ~5200 IIRC. You could try simplifying your folder structure to see if that would help.

The sandisk firmware is what it is. No fixes there. Rockbox would be the only alternative.